L-Histidinol, an analogue of the essential amino acid L-histidine, elicits differential growth responses from a variety of 'normal' and of 'transformed' (oncogenic) cells. Normal cells are arrested or maintained in a Go-like state whereas their transformed counterparts show persistent, but compromised, cell-cycle transit. The differential growth response in general, improves the therapeutic index of various anticancer drugs in culture and provides a basis for a combined genetic/biochemical approach for investigation perturbations in growth control mechanisms which attend oncogenic transformation. On-going studies to evaluate the possibility that a histidinol/anticancer drug approach might improve cancer therapy in vivo, as well as an analysis of growth control mechanisms characteristic of normal and transformed cells, continue.